Apple is "extremely cheap" because "the stock will be at $600 [again] in the next 18 months," Howard Ward, Gamco CIO for Growth Equities, told CNBC on Thursday.

The iPhone maker was the darling of the first nine months of 2012 — soaring 65 percent to an all-time high of about $705 a share. But since September's peak, the stock is down more than 30 percent — half of which was lost since the beginning of this year.

Ward was undeterred in a "Squawk Box" interview, saying that investors should "buy it again or buy it."

"This is a company that's made more money for shareholders in the last few years than the entire rest of the technology industry combined," he said.

"At 4.7 times earnings before interest, taxes, depreciation, and amortization (EBITDA: CNBC Explains), a 50 percent discount to IBM — which has grown revenues by 1 percent for the past five years — Apple is extremely cheap."

Apple has been under pressure recently from hedge fund manager David Einhorn, who spoke on "Squawk Box" two weeks ago about his call for a distribution of perpetual preferred stock. Now he's taking his case directly to investors to explain why he's pushing the company to share its treasure trove of cash.